Is it okay to leave a 16 year old...?

Is it okay to leave your 16 year old daughter with her 6 year old sister from 7 until like 2-3am every weekend, while the parents go out with friends to bars, clubs, boat parties, ect. The mother ALWAYS comes home high, and drunk. The father doesn't always get high, but gets drunk/drinks a lot. They do this every weekend, and I understand that adults should get to have fun sometimes, but something about this just feels wrong are irresponsible to me.